In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plates, also called printing plate precursors, typically comprise a top layer applied over the surface of a hydrophilic substrate. The top layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material.
If after exposure to radiation, the exposed regions are removed in the developing process, revealing the underlying hydrophilic surface of the substrate, the plate is called a positive-working printing plate. Conversely, if the unexposed regions are removed by the developing process and the exposed regions remain, the plate is called a negative-working plate. In each instance, the regions of the radiation-sensitive layer (i.e., the image areas) that remain repel water and accept ink, and the regions of the hydrophilic surface revealed by the developing process accept water, typically a fountain solution.
Direct digital imaging of offset printing plates, which obviates the need for exposure through a negative, is becoming increasingly important in the printing industry. Positive working, multi-layer, thermally imageable elements that comprise a hydrophilic substrate, an alkali developer soluble underlayer, and a thermally imageable top layer have been disclosed. On thermal imaging, the exposed regions of the top layer become soluble in or permeable by the alkaline developer. The developer penetrates the top layer and removes the underlayer and the top layer, revealing the underlying substrate. Such systems are disclosed in, for example, Parsons, U.S. Pat. No. 6,280,899; Shimazu, U.S. Pat. No. 6,294,311, and U.S. Pat. No. 6,352,812; and Savariar-Hauck, U.S. Pat. No. 6,358,669.
Despite the advantages that have been made in the development of multi-layer thermally imageable elements, elements in which the top layer has increased resistance to damage during handling would be desirable. The top layer of a multi-layer, thermally imageable element is sensitive to mechanical damage. It may, for example, be easily scuffed or scratched away when the imageable element is transported with suction cups in a platesetter or when it is transported to a customer location. Because of the low coating weight for the top layer (about 0.7 g/m2), a shallow scratch is sufficient to break through the thin top layer. Because the underlayer is readily soluble and/or penetrable by the developer, the regions of the underlayer exposed by the scuffs and scratches will be removed by the developer. The plate rejection rate for multi-layer thermally imageable elements due to this failure mode can be high relative to that for single layer, thermally imageable elements, in which the top layer is much thicker. Thus, a need exists for positive working, multi-layer, thermally imageable elements that have increased resistance to damage during handling.